Hahahaha actually is the exact polor oppossite. Thats an assumption that most people wrongly make without really understanding the true way, hid lights and concrete work together. This is one of those times where theroy and actual application are miles apart. Concrete is a conductor of both heat and cold. It will also absorb water. So without a heat source a concrete slab will remain frigidly cold. On the other hand, with a heat source, like say......for example.........4000 watts of bare bulb HID lighting running 24/0 that same concrete slab does and will absorb and retain the heat put off by the lights, even in the dead of winter when its freezing out side. Obviously if you have a concrete slab and HID lights above it running 24/0 it would be quite silly to think the concrete would be cold and the roots unhappy. The reality is that, roots like warm. So If you can imagine a huge smooth and flat rock the size of a car on a beach getting hit with perfect 78-80 degree summer sunshine all day long for a month strait, how nice and warm the rock would be. And how nice, warm, comfy and relaxing it would feel to lay on that rock......my roots feel the same. so the issue its caused me now is accelerated root growth to the point where the roots have burst out of the bottom of the pots and they are growing out onto the concrete slab as they love the warmth and heat it puts off so it's cause me to become root bound earlier in veg than I normally would and they outgrow their pots sooner than they normally would and the veg growth has been quicker then it normally would be. Roots love warm concrete floors and thats why we run 24/0 veg light cycles during winter when growing on a concrete floor. 18/6 or 20/4 Allows a break in the heat source and gives the concrete the opportunity to now be a conductor of the cold outside temps and cool down and Roots dont like cold, as we all know. So I understand why you would " think " they wouldn't like it. But theroy and application yeild far different results in life and this is a great example of that.